1. Field of the Invention
The present invention relates generally to apparatus for inspecting the surface of non-flat objects, and more specifically to apparatus for producing a video signal of the object for use in determining the quality of the object.
2. Description of the Prior Art
Various surface inspection apparata that use a television camera as the sensor to detect whether defects such as surface flaws exist have been proposed.
One of the television camera arrangements used at present in such systems is explained with reference to FIG. 1. In FIG. 1, a television camera used as a sensor is provided with an optical system 2 such as a convex lens and a target screen spaced along the optical axis 4. The image forming function of the optical system as used in the normal television camera 1 is, as is commonly known, able to form a clear and accurate image of only the specific area of an object 7 lying at a point P on the optical axis 4. Further, even in the event that the optical system 2 can be adjusted relative to the plane 5, it is well known that due to the so-called focal depth of the aperture stop of the optical system 2, the portion of the image within the range 6, i.e., between planes 5.sub.1 and 5.sub.2 (as shown by broken lines), each being parallel to plane 5 can be imaged substantially in focus. That is, points P1 and P2 before and after the point P on the optical axis 4, will be formed on the target screen 3 in substantially clear image. Therefore, television camera 1 is able to generate effective video signals of the objects that are within range 6.
However, if it is to be assumed that the surface 7a of the object 7 to be inspected is curved in a concave form, for example, as shown in FIG. 1 in relation to the television camera, the surface 7a includes portions that stick out on either side of the above mentioned range 6. If the conventional optical system 2 is focused to plane 5, the images of these extended portions will not be formed clearly on the target screen 3 resulting, therefore, in a so-called out-of-focus image. Accordingly, television camera 1 is not able to generate effective video signals for the extended portions at the same time that the portions within the range 6 are inspected.
It is possible to reduce the extended portion by reducing the aperture stop of the optical system 2 by which the focal depth thereof is made deeper enabling the range 6 to be enlarged behind and in front of the plane 5, but there is a limit to how far the range 6 can be spread. Depending upon the degree of curvature of the inspected surface 7a, there are many occasions where not all of the extended portions can be covered by adjusting the aperture stop of the optical system 2. Further, such reduction of the aperture stop of the optical system 2 causes a reduction of the amount of light that passes through the optical system 2. Accordingly, the amount of light that is incident on the target screen 3 of the television camera 1 is reduced, which causes the video signals to be so weak that inspection is not feasible. To avoid this, the amount of light from a light source (not indicated on the drawing) that irradiates the inspected surface 7a must be increased. However, there is also a limit to the extent to which the light may be increased so that a full concurrence to such above mentioned degree of aperture stop close down can be made.
On the other hand, it may be considered that by mounting on the television camera, a lens with a long focal length, such as a telescopic lens, the entirety of the curved inspected surface may be formed as an effective image on the target screen of the television camera without any out of focus portion. However, as is well known, when a telescopic lens is used, its focal length is so long that the television camera must be placed at some distance from the inspected object. Therefore, the use of telescopic lens on the television camera gives rise to problems of space, light source and further, due to the long focal length, as well as large disadvantageous influence arising from the shaking motions of the television camera.
In FIG. 2, another prior art arrangement is shown wherein an optical system such as convex lens 2 is used on the conventional television camera 1 so that it forms an image of a concave object 7 as an image 7' which simulates the concave object 7 although curved opposite to the object 7 as shown in FIG. 2.
In such case, if the focus of lens 2 is matched to the deepest part 0 of the object 7, the images of the other portions of the object 7 (closer portions to lens 2) will be out of focus if the focal depth is not considered. In other words, if a screen (target screen 3 of the television camera 1) is placed perpendicularly to the optical axis 4 at point 0' which corresponds to the deepest point 0 of the object 7 on the optical axis 4, the image at point 0' on the screen will be in focus, but the other portions will be all under the so-called out-of-focus condition. On the contrary, when the lens 2 focuses on the object 7 at its shallowest (foremost) portion of the image 7', at least the image at point 0' becomes out of focus.
Accordingly, the conventional surface inspection apparata that use optical systems such as those mentioned above have inevitable problems upon inspection of the non-flat surface, so that not all of the surface can be inspected at the same time.